State works to suppress West Nile virus

Rick Bogren | Aug 30, 2002

They swarm. They bite. They suck your blood. And they spread disease. They're mosquitoes, and with more than 65 species in the state, Louisiana has more types of these insects than most other states in the United States.

“We're subtropical, their preferred climate,” says Dr. Michael Perich, a medical entomologist and internationally recognized mosquito expert with the LSU AgCenter. “One-third of the species in Louisiana are the same as in Brazil.”

Louisiana has nearly every mosquito habitat, Perich says, pointing out the state is home to fresh marsh, salt marsh, floodwater, rice field, urban and even tree-hole mosquitoes — a variation that lays its eggs in small pools of water found in holes in trees.

The south Louisiana habitat mimics tropical zones, creating conditions that support mosquitoes year-round, Perich points out. Some species actually thrive when temperatures are in the 50s and 60s — not uncommon as the high temperatures for south Louisiana in winter.

Among all the mosquitoes in Louisiana, the two principal species found where people live are the Southern house mosquito — Culex quinquefasciatus — and the Asian tiger mosquito — Aedes albopictus.

The Southern house mosquito, sometimes called the “sewer mosquito,” breeds in organically rich areas such as drainage ditches and septic ponds, Perich says. Although this species prefers to bite birds, it will feed on people, too.

“Birds are their steak; we're more like hot dogs,” Perich says.

But the LSU AgCenter expert says the No. 1 nuisance mosquito in Louisiana is the Asian tiger mosquito.

“It's very domesticated,” he says. “It lays eggs at the edge of water and breeds in backyard containers such as bird baths, flower pot saucers, swimming pool covers, boat covers and even in flowers such as bromeliads.”

The Asian tiger mosquito came to the United States in used tires from Asia.

“It's a vector (carrier) of about everything you can imagine, and it feeds on everything,” Perich says, adding that not all mosquito species bite people.

“Mosquitoes can be very host-specific,” the entomologist says. “Culiseta melanura, for example, bites birds exclusively. One species doesn't feed on blood at all, but on other mosquitoes.”

Even the mosquitoes that feed on blood don't do that to live, Perich says. For “regular food,” both male and female mosquitoes consume nectar as their main energy source.

Blood, on the other hand, contains the ingredients necessary to lay eggs. That's why only females bite.

Most female mosquitoes live about one week, Perich says. They bite animals — birds, people, dogs, horses and so forth — to get a “blood meal” to lay eggs.

Perich says the female mosquito has a three-day cycle between the blood meal and laying the eggs. Because they live only seven to 10 days, female mosquitoes will do this only two to three times in their lifetimes.

Besides old age, the No. 1 killer of mosquitoes is the sun, Perich says. That's why most mosquitoes feed at night.

But there are a few exceptions. Woodland mosquitoes can be active nearly all day because they live in deeply shaded areas. And the Asian tiger mosquito, which can carry West Nile virus, feeds during the daylight — generally from dawn to about 9 a.m. and again from about 4 p.m. until dusk.

Although most mosquitoes don't venture very far from their birthplace, marshland mosquitoes can travel as far as 12 miles looking for water and blood meals in dry weather. During these times, salt marsh and floodwater mosquitoes can be in large swarms that in some areas of the world have been known to kill young animals by biting them so often they bleed them to death.

Perich says mosquitoes can find you in several ways. Some of the cues are visual. Mosquitoes, like other insects, see in the blue, ultraviolet spectrum. So they're attracted to blue lights and clothing, but not to reds and yellows. That's why bug zappers have blue lights and why some people use yellow lights on porches.

Because they're often out at night when they can't see well, mosquitoes also are attracted by heat and aromas. This explains why some people are more attractive to the insects. They may “smell better” or have a slightly higher body temperature.

Finally mosquitoes are attracted to carbon dioxide — the stuff that comes out of your mouth when you breathe. “You can breathe,” Perich jokes. “Just don't exhale.”

Because of the way they feed on blood, female mosquitoes spread disease by picking up a disease from one animal and then passing it on to another, including a person or horse.

Most of these diseases are viruses, Perich says. And these viruses have to go through an incubation period — generally three to seven days.

To transmit a disease, the mosquito must get it from her first victim and after the incubation period, pass it on to another victim — probably the last one she bites before she dies.

Perich says the Southern house mosquito is the primary vector — or carrier — for St. Louis encephalitis and West Nile virus. While also the vector for West Nile, the Asian tiger mosquito is a vector for dengue fever and other diseases, too.

Viruses are commonly named after the area where they're originally discovered, Perich explains. For example, LaCrosse virus, which is carried by tree-hole mosquitoes and primarily affects children younger than one year old, was discovered in LaCrosse, Wis.

But not all mosquito-borne diseases are viruses, Perich cautions. Malaria, for example, is caused by a parasite that mosquitoes can carry just as they do viruses.

Travelers — including people or migratory birds — can pick up a virus or parasite and bring it into the United States, where it can be transmitted by mosquitoes.

For example, Perich says the West Nile virus is an arbovirus — it's carried by birds. It's hard to predict and appears to occur in cycles, he says.

Some birds, such as blackbirds, blue jays and northern cardinals, die from West Nile virus. Others, such as house sparrows, don't.

“The way to attack West Nile is to suppress cases and shorten the season,” Perich says. “You can't eradicate the disease, but you can suppress it.”

Perich says mosquito abatement districts are important parts of mosquito control programs. They can spray to keep populations down and control the mosquitoes so they aren't as big a problem. The down side is that when control programs are effective, people don't notice because the disease doesn't appear.

“West Nile virus will always be here, but with control it will become less of a problem,” Perich says.

Malaria, dengue fever and yellow fever are more threatening than West Nile virus, Perich says, pointing out that yellow fever was last diagnosed in the United States in New Orleans in 1900.

Perich, who has suffered malaria, dengue fever and other mosquito-borne diseases, spent 15 years in preventive medicine for the U.S. Army before joining the LSU AgCenter.

“I've had it. I've seen it. I'm a little more driven,” he says of his experiences and his desire to work with mosquito-borne diseases.

“I've seen people die,” he adds. “I had a 13-year-old girl die of dengue hemorrhagic fever in my arms in Thailand. She was the age of my daughter.”

Perich works closely with the Centers for Disease Control and Prevention, the World Health Organization, the U.S. Agency for International Development and similar organizations around the world.

Part of his research is funded by international agencies and corporate sponsors.

The Nebraska native earned his bachelor's degree from Iowa State University and his master's and doctoral degrees from Oklahoma State University. He joined the LSU AgCenter faculty in 2001.